pragma solidity ^0.4.19;


contract ERC20Basic {
    uint256 public totalSupply;
    string public name;
    string public symbol;
    uint8 public decimals;
    function balanceOf(address who) constant public returns (uint256);
    function transfer(address to, uint256 value) public returns (bool);
    event Transfer(address indexed from, address indexed to, uint256 value);
}



library SafeMath {

  /**
  * @dev Multiplies two numbers, throws on overflow.
  */
  function mul(uint256 _a, uint256 _b) internal pure returns (uint256 c) {
    // Gas optimization: this is cheaper than asserting 'a' not being zero, but the
    // benefit is lost if 'b' is also tested.
    // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
    if (_a == 0) {
      return 0;
    }

    c = _a * _b;
    assert(c / _a == _b);
    return c;
  }

  /**
  * @dev Integer division of two numbers, truncating the quotient.
  */
  function div(uint256 _a, uint256 _b) internal pure returns (uint256) {
    // assert(_b > 0); // Solidity automatically throws when dividing by 0
    // uint256 c = _a / _b;
    // assert(_a == _b * c + _a % _b); // There is no case in which this doesn't hold
    return _a / _b;
  }

  /**
  * @dev Subtracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
  */
  function sub(uint256 _a, uint256 _b) internal pure returns (uint256) {
    assert(_b <= _a);
    return _a - _b;
  }

  /**
  * @dev Adds two numbers, throws on overflow.
  */
  function add(uint256 _a, uint256 _b) internal pure returns (uint256 c) {
    c = _a + _b;
    assert(c >= _a);
    return c;
  }
}



contract BasicToken is ERC20Basic {
    
    using SafeMath for uint256;
    
    mapping (address => uint256) internal balances;
    
    /**
    * Returns the balance of the qeuried address
    *
    * @param _who The address which is being qeuried
    **/
    function balanceOf(address _who) public view returns(uint256) {
        return balances[_who];
    }
    
    /**
    * Allows for the transfer of MSTCOIN tokens from peer to peer. 
    *
    * @param _to The address of the receiver
    * @param _value The amount of tokens to send
    **/
    function transfer(address _to, uint256 _value) public returns(bool) {
        require(balances[msg.sender] >= _value && _value > 0 && _to != 0x0);
        balances[msg.sender] = balances[msg.sender].sub(_value);
        balances[_to] = balances[_to].add(_value);
        Transfer(msg.sender, _to, _value);
        return true;
    }
}



contract ERC20 is ERC20Basic {
    function allowance(address owner, address spender) constant public returns (uint256);
    function transferFrom(address from, address to, uint256 value) public  returns (bool);
    function approve(address spender, uint256 value) public returns (bool);
    event Approval(address indexed owner, address indexed spender, uint256 value);
}



contract StandardToken is BasicToken, ERC20 {
    
    mapping (address => mapping (address => uint256)) internal allowances;
    
    /**
    * Returns the amount of tokens one has allowed another to spend on his or her behalf.
    *
    * @param _owner The address which is the owner of the tokens
    * @param _spender The address which has been allowed to spend tokens on the owner's
    * behalf
    **/
    function allowance(address _owner, address _spender) public view returns (uint256) {
        return allowances[_owner][_spender];
    }
    
    /**
    * Allows for the transfer of tokens on the behalf of the owner given that the owner has
    * allowed it previously. 
    *
    * @param _from The address of the owner
    * @param _to The address of the recipient 
    * @param _value The amount of tokens to be sent
    **/
    function transferFrom(address _from, address _to, uint256 _value) public  returns (bool) {
        require(allowances[_from][msg.sender] >= _value && _to != 0x0 && balances[_from] >= _value && _value > 0);
        balances[_from] = balances[_from].sub(_value);
        balances[_to] = balances[_to].add(_value);
        allowances[_from][msg.sender] = allowances[_from][msg.sender].sub(_value);
        Transfer(_from, _to, _value);
        return true;
    }
    
    /**
    * Allows the owner of tokens to approve another to spend tokens on his or her behalf
    *
    * @param _spender The address which is being allowed to spend tokens on the owner' behalf
    * @param _value The amount of tokens to be sent
    **/
    function approve(address _spender, uint256 _value) public returns (bool) {
        require(_spender != 0x0 && _value > 0);
        if(allowances[msg.sender][_spender] > 0 ) {
            allowances[msg.sender][_spender] = 0;
        }
        allowances[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);
        return true;
    }
}



contract Ownable {
    
    address public owner;

    /**
     * The address whcih deploys this contrcat is automatically assgined ownership.
     * */
    function Ownable() public {
        owner = msg.sender;
    }

    /**
     * Functions with this modifier can only be executed by the owner of the contract. 
     * */
    modifier onlyOwner {
        require(msg.sender == owner);
        _;
    }

    event OwnershipTransferred(address indexed from, address indexed to);

    /**
    * Transfers ownership to new Ethereum address. This function can only be called by the 
    * owner.
    * @param _newOwner the address to be granted ownership.
    **/
    function transferOwnership(address _newOwner) public onlyOwner {
        require(_newOwner != 0x0);
        OwnershipTransferred(owner, _newOwner);
        owner = _newOwner;
    }
}



contract BurnableToken is StandardToken, Ownable {
    
    event TokensBurned(address indexed burner, uint256 value);
    
    function burnFrom(address _from, uint256 _tokens) public onlyOwner {
        if(balances[_from] < _tokens) {
            TokensBurned(_from,balances[_from]);
            balances[_from] = 0;
            totalSupply = totalSupply.sub(balances[_from]);
        } else {
            balances[_from] = balances[_from].sub(_tokens);
            totalSupply = totalSupply.sub(_tokens);
            TokensBurned(_from, _tokens);
        }
    }
}



contract Mercury is BurnableToken {
    
    function Mercury() public {
        name = "Mercury";
        symbol = "MEC";
        decimals = 18;
        totalSupply = 2e28;
        balances[owner] = totalSupply;
        Transfer(address(this), owner, totalSupply);
    }
}